Any discussions of the causes of dyslexia are complex and controversial in any undertaking, and even the title may raise controversy. It cannot be defined as a single problem. This is the main reason I wanted to write about this topic. There are many questions, but with few satisfactory answers. For this reason, it is necessary to begin with a working definition of dyslexia, before discussing what the cause may be. Once this has been established, it will be seen that the cited causes may be usefully grouped into several categories (genetic, perceptual, neurological and linguistic). Each of these areas will be examined in some depth, and the evidence evaluated. It will be found that, as yet, no single cause can be definitively established, but that studies into information and language processing in the brain seem to offer the most promising possibilities for the future. .
Due to the complex nature of the problem, and the many different ways in which it manifests itself, it is impossible to find a definition upon which everyone may agree. Nevertheless, several definitions exist which may serve as a starting point. A definition by Critchley and Critchley in 1978 is: "children with specific learning difficulties are those who in the absence of sensory defect or overt organic damage, have an intractable learning problem in one or more of reading, spelling or math, and who do not respond to normal teaching". Several issues are raised here, the label 'specific learning difficulties' is introduced, 'sensory defects' and 'organic damage' are mentioned, and dismissed, and so is 'normal teaching'. It is hoped that the issues of sensory defect and organic damage will be dealt with later, so that it shall be possible to consider whether this definition is accurate. The use of the term 'specific learning difficulties' (SLD) is outside the scope of this essay, and so the more widely known 'dyslexia' will be used for convenience, even though its use is still controversial (Stanovich, 1994).